There are many factors that have to be addressed in the design of bearings for high-speed ( rpm) turbine rotors for Steam Turbine-Generators. Among these factors are the following: Vertical forces from rotor weights and vertical misalignments across couplings; Lateral forces on the High-Pressure turbine rotors from steam emanating from the entrance nozzles from the &#;Nozzle Block&#;; temperatures of the journals, temperatures of the housings surrounding the bearings, and even the temperature distribution throughout the foundation for a turbine-generator.
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It is clearly not satisfactory to assume that the only weight to be used in calculating bearing performance is the cold static weight carried by the bearing when the centerline of the rotor train has the shape of the ideal catenary from one end of the train to the other end. This is one case, and may be the first case studied, but others need to be considered depending upon the circumstances of the application. The designer must be well aware of all of the forces acting on the bearings, the temperature distributions through the machine, both transient and static, and other influences such as the range of lube oil temperatures expected.
TRI Align-A-Pad® bearings are extremely effective for High Pressure and the Intermediate Pressure Rotors with journals from 6 inches to 20 inches. The typical designs are 5-pad or 6-pad configurations depending upon bearing reaction forces and speeds.
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Many existing Low Pressure Turbine Bearings are fixed bore with a spherical OD fitted to an outer bearing ring that has &#;saddle blocks&#; or alignment pads on the OD. If the turbine rotors show a tendency to exhibit vibrations of substantial amplitude and are unreliable when being balanced, a choice is to redesign the bore geometry to stiffen the film and to suppress the rotor vibration amplitudes.
It should be noted that today, the price for a new TRI Align-A-Pad® Bearing which has self-aligning pads and has &#;saddle blocks&#; or alignment pads on the OD is almost the same as for a new fixed bore bearing with spherical OD and an outer bearing ring with &#;saddle blocks&#; or alignment pads on the OD. This is how an improved technology can be very cost effective for new or refurbished machines.
An issue for the HP and IP turbines that have very hot steam conditions is the heating of the standards adjacent to the turbines. When the standards get hot, they expand more than the bearings that are being cooled by colder lube oil, so the bearings can become loose. TRI has numerous methods to design bearings to remain tightly mounted in standards that get hot.